I've never been a big fan of SLI unless you are running massive resolutions, ie multi-monitor setups. Since you are running a pretty standard resolution for today's market, I would suggest upgrading to a single better card. It'll save you a short term headache of having to build only half a system and maintain compatibility with other components you currently have.

SLI also creates quirks every once in a while. Most modern games should be fine on it, because they test for it now. But even still, I've seen new games come out with strange SLI issues. It's just overall simpler to have 1 fast card over 2 medium cards. Plus, you always know your card is running at max capacity.